[Clang][Sema] Fix out-of-bounds access (#80978)

Trying to compile a C-style variadic member function with an explicit
object parameter was crashing in Sema because of an out-of-bounds
access.

This fixes #80971.

Author: Sirraide

clang/docs/ReleaseNotes.rst

@@ -208,6 +208,8 @@ Bug Fixes to C++ Support
   parameter where we did an incorrect specialization of the initialization of
   the default parameter.
   Fixes (`#68490 <https://github.com/llvm/llvm-project/issues/68490>`_)
+- Fix a crash when trying to call a varargs function that also has an explicit object parameter.
+  Fixes (`#80971 ICE when explicit object parameter be a function parameter pack`)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaOverload.cpp

@@ -7722,8 +7722,8 @@ bool Sema::CheckNonDependentConversions(
   unsigned Offset =
       Method && Method->hasCXXExplicitFunctionObjectParameter() ? 1 : 0;
 
-  for (unsigned I = 0, N = std::min(ParamTypes.size(), Args.size()); I != N;
-       ++I) {
+  for (unsigned I = 0, N = std::min(ParamTypes.size() - Offset, Args.size());
+       I != N; ++I) {
     QualType ParamType = ParamTypes[I + Offset];
     if (!ParamType->isDependentType()) {
       unsigned ConvIdx;

clang/test/SemaCXX/cxx2b-deducing-this.cpp

@@ -636,3 +636,13 @@ struct D {
     }
 };
 }
+
+namespace GH80971 {
+struct S {
+  auto f(this auto self...) {  }
+};
+
+int bug() {
+  S{}.f(0);
+}
+}